Delta-sigma modulators (noise shapers) are particularly useful in digital to analog and analog to digital converters (DACs and ADCs). Using oversampling, a delta-sigma modulator spreads quantization noise power across the oversampling frequency band, which is typically much greater than the input signal bandwidth. Additionally, a delta sigma modulator performs noise shaping by acting as a lowpass filter to the input signal and a highpass filter to the noise; most of the quantization noise power is thereby shifted out of the signal band.
In addition to data conversion applications, delta-sigma noise shapers are increasingly utilized in the design of digital amplifiers. In one particular technique, a digital delta-sigma noise shaper provides a noise shaped (quantized) digital data stream to a pulse width (duty cycle) modulated (PWM) stream, which in turn drives a linear amplifier output stage and associated load. This technique is generally described in U.S. Pat. No. 5,784,017, entitled “Analogue and Digital Convertors Using Pulse Edge Modulators with Non-linearity Error Correction”, granted Jul. 21, 1998, and U.S. Pat. No. 5,548,286, entitled “Analogue and Digital Convertor Using Pulse Edge Modulators with Non-linearity Error Correction”, granted Aug. 20, 1996, both to Craven, U.S. Pat. No. 5,815,102, entitled “Delta Sigma PWM DAC to Reduce Switching”, granted Sep. 29, 1998, to the present inventor (incorporated herein by reference), U.S. patent application Ser. No. 09/163,235 to Melanson (incorporated herein by reference), and International Patent Application No. PCT/DK97/00133 by Risbo.
One problem, which occurs in PWM circuits driving full-bridge loads, is noise and distortion caused by the non-zero impedance of the voltage supply. In particular, for a full-bridge output, a pair of drivers, typically operating from a single voltage supply, is utilized to drive a corresponding pair of output signal paths coupled to the full-bridge output load. Glitches on the output signal paths occur when the outputs of the two drivers switch simultaneously or nearly simultaneously. Specifically, the output of one-driver transitions towards the power supply voltage and the output of the other driver transitions towards ground. Due to the non-zero impedance of the voltage supply, the output paths do not settle to their final state instantaneously, and glitches are generated as an intermediate voltage appears across the corresponding outputs.
One approach to driving in a full-bridge output of a PWM system is disclosed in U.S. Pat. No. 6,373,336 to Anderskouv et al., and entitled Method Of Attenuating Zero Crossing Distortion And Noise In An Amplifier, An Amplifier And Uses Of The Method And The Amplifier, issued Apr. 16, 2002 (hereinafter the Anderskouv system). In this system, each terminal of a full-bridge output load is driven by a different PWM encoded signal provided by a corresponding separate PWM processing path. One processing path processes an input data stream, while the other processing path processes the complement of the input data stream. These complementary data streams drive a corresponding pair of delta-sigma modulators. Except for small levels of the input signal, when the input stream and its complement are close in value, the delta-sigma modulators generate substantially different modulated streams. The modulated data streams drive corresponding separate PWM modulation stages, which in turn drive the terminals of the full-bridge output loads.
Disadvantageously, the Anderskouv system does not guarantee that the outputs of the PWM modulators will not switch simultaneously or nearly simultaneously. In particular, for small values of the input signal, the outputs of the PWM stages of the Anderskouv system will switch nearly simultaneously. This near simultaneous switching will cause power supply glitches in the output signal, which will not be masked by the corresponding small output signals. Another significant disadvantage of the Anderskouv system is its hardware inefficiency, since two PWM paths, each including a PWM encoder, are required. This hardware inefficiency is particularly disadvantageous when utilized in multi-channel signal processing systems, such as those required in advanced audio applications, such as home theater audio.
Hence, given the increased utilization of PWM systems in such applications as audio signal processing, new, efficient, techniques are required for minimizing distortion and noise in full-bridge outputs driven by PWM—encoded data.